I thought I'd provide some thoughts on this, since I've been doing both over the years.

What I would say, is that both address different areas, and both are required for a balanced approach towards therapy.

ACT is really good at dealing with suffering and things like "unwanted thoughts". This is where I think CBT kind of fails, or at least isn't very effective, or sustainable.

On the other hand, where ACT falls apart is when it comes to pursuing valued actions. It's a very good framework for dealing with suffering, but terrible when it comes to whole "what next" question. It just doesn't provide much there.

This is where I think CBT come in, because it teaches you to look at things in an optimistic way, which is how you want to approach your valued action. It teaches you how to thrive, instead of just not suffer.

Would love to hear your thoughts.